With advancements in technology, the transmission of voice and data at faster rates and in larger volumes is always in demand. One solution to fulfilling these demands is digital subscriber line (DSL) technology. DSL technology has been introduced into the field of broadband networking, among other reasons, to overcome issues faced by traditional voice band technology. Such issues include, but are not limited to, bandwidth limitations. Multiple DSL technologies exist including, but not limited to, rate adaptive DSL (RADSL), symmetric DSL (SDSL), multi-rate SDSL (M/SDSL), high bit-rate DSL (HDSL), very high bit-rate DSL (VDSL), and asymmetric DSL (ADSL).
ADSL technology utilizes the infrastructure already in place in a public switched telephone network (PSTN), including copper loops constructed of copper wires, between a customer premise and a central office. Advantageously, ADSL technology does not require replacement of network equipment such as routers, switches, firewalls and Web servers, which are commonly used in today's paradigm for broadband access.
The transmitting and receiving of information on a common wire creates coupling and interference between the transmitted and received signal. This interference is referred to as echo. One of the characteristics of echo is that a transmit waveform that is transmitted within a local transmit path, is coupled to a local receive path, thereby interfering with received signals. The result of this reflection is a differently shaped waveform than was originally intended to be received, which introduces data degradation.
Echo cancellation is the elimination of the transmit signal from the received signal. Numerous methods have been used for removing transmit echo. One such method, for example, comprises use of a least mean squared adaptation. Least mean squared adaptation finds a correlation between a received signal and error caused by echo. Specifically, an echo canceler is utilized to cancel transmit echo, thereby leaving a clean incoming signal.
The echo is generated by two types of error, linear error as well as non-linear error. Many of today's current applications can correct for the linear error. Digital Signal Processors (DSP) can easily detect the differences between the receive signal and the transmit signal it sent as long as the differences are linear. If the differences can be detected, an echo canceler can easily eliminate the error. However, DSPs can not easily recognize non-linear error.
Non-linear errors are caused by non-linearities present in components that process the transmit and receive signals. In particular, a line driver generates much of the non-linear error on the transmit signal. A line driver drives the transmit signal through a two-wire pair in accordance with a required power particular to the application considered and the type of line driven. Most established DSL applications have a required standard power spectral density template that they are required to meet. As an example, for the most common DSL service currently deployed (2B1Q HDSL), the required transmit power is approximately 13.5 dBm. This yields a maximum peak power of 18.6 dBm, for a peak current of 48.2 mA (assuming 6Vpp differential output voltage, 3 dB loss in back matching resistors, and a peak-to-average ratio (PAR) of 1.8 for 2B1Q).
At the other extreme, the required transmit power for a Discrete Multi Tone-Asymmetric Digital Subscriber Line-Central Office (DMT-ADSL-CO), assuming 3 dB loss in the back matching resistors and a PAR of 5.3 for DMT, is approximately 20.5 dBm. This yields a maximum peak power of 37.98 dBm, for a peak current of 2095 mA (assuming a 6Vpp differential output voltage). It should be noted that there is a multitude of other applications at various power levels between these two extremes. Also, due to line impairment caused by a variety of different factors, particularly bridge taps, the actual line impedance might be significantly less then expected, and the load current can thus be significantly higher then expected.
For DSL systems, the line driver is typically the most significant source of distortion in the data transmit path, due to the high speed and large load current requirement, combined with the variety of line configurations encountered. For most high-speed ADSL applications the trend is currently to avoid transmitting and receiving in the same frequency bands. From the customer premise side, typically there is transmission at the low frequency end of the band and reception at the upper frequency end of the band. Thus, distortion products, which fall at multiples of the transmit signal, fall in the receive spectrum. Typically, distortion in the transmit path couples into the receive path through a hybrid, which makes the receiver implementation significantly more challenging. The receiver recovers the echoed portion of the transmit signal, which falls in the receive band, as well as the receive signal itself. Therefore, limitation of distortion in the transmit path is important to both the transmission and reception of information.
It should also be noted, that distortion introduced in the receive band by any element in the transmit path, up to the line driver, can be filtered out with a passive external LC filter before it is fed to the line driver. However, if the line driver introduces the distortion, it is not practical to filter the distortion out due to the low impedance level. Thus, the distortion introduced by the line driver is the final limiting factor for the distortion of the transmit path, which typically is the most critical factor in terms of achievable reach of the system.
Generally, line drivers are implemented with high input impedance amplifiers. This makes it easy to interface to the line driver. However, it implies that there is significant signal swing across the input terminals of the line driver. This changes the common mode input to the line driver, which inherently changes the gain, in turn, yielding distortion and degradation of both transmitted and received data.
A system and method is desirable to not only eliminate the linear errors that crossover from the transmit path to the receive path, but to also eliminate non-linear errors generated by components that process the transmit signal. In particular, the errors caused by the line driver.